This invention relates to a magnetic storage apparatus and magnetic recording media and, more in particular, it relates to a magnetic storage apparatus having an areal recording density of 10 Gbits or more per square inch and magnetic recording media for providing the same.
In recent years, amount of information processed in computers has been increased more and more and greater capacity and higher speed transfer have been demanded for a magnetic disk apparatus as an external storage device. At present, magnetic disk apparatus having a recording density in the order of 4 Gbit per one square inch at the maximum have been put into practical products. For a magnetic head in such high density magnetic disk apparatus, a composite type head of separating a recording portion and a reproducing portion and using an inductive head for the recording portion and a magnetoresistive head for the reproducing portion has been adopted.
Since the magnetoresistive head is highly sensitive not only to signals but also noises of media, lowering of noises has been demanded more for the magnetic recording media than usual. Medium noises in the longitudinal magnetic recording system are mainly caused by disorder of magnetization in a magnetization transition region between recorded bits and narrowing of the region leads to the lowering of the medium noises. For this purpose, it is effective to decrease the magnetization reversal size by decreasing the size of magnetic grains and weakening the intergranular interaction. The magnetic grains can be made fine decreasing the size of by refining underlying grains by utilizing an epitaxial relation between a magnetic layer and an underlayer. Further, the intergranular interaction can be weakened by segregating non-magnetic Cr to the grain boundary and, for promoting the segregation of Cr, studies have been made for CoCrPt alloys at high Cr concentration, CoCrPtTa alloys and CoCrPtB alloys.
For coping with linear recording density increasing year by year, it is necessary to make the coercivity (Hc) of magnetic recording media higher. It is necessary to increase the Pt concentration in order to obtain high Hc in a CoCrPt alloy at high Cr concentration, but the overwrite characteristics tend to lower abruptly as the Pt concentration increases. On the other hand, in the CoCrPtTa alloy, since grain boundary segregation of Cr occurs even when the Cr concentration is not so high and magnetic grains show high magnetic isolation, high Hc can be obtained at a relatively low Pt concentration. However, also in the case of using the CoCrPtTa alloy, the Cr concentration has to be increased for further decreasing medium noises and the Pt concentration has to be increased in order to ensure high Hc with finer grain size. However, in the CoCrPtTa alloy, epitaxial growth of the underlayer and the magnetic layer is difficult at composition of high Cr concentration, and Pt concentration to result in a problem that no sufficient magnetic characteristics and read/write characteristics can be obtained. Such a problem is also observed in a case of using a CoCrPtB alloy magnetic layer with high Pt concentration.
This invention has been made in order to solve the foregoing subject. More specifically, this invention intends to provide magnetic storage apparatus having high S/N ratio at a recording density of 10 Gbits or more per square inch and excellent in the reliability.
The foregoing object can be attained in accordance with this invention in a magnetic storage apparatus including a magnetic recording medium having, on a substrate, a seed layer, one or plurality of underlayers formed on the seed layer and one or plurality of magnetic layers formed on the underlayer, a driver for driving the medium in the recording direction, a read/write separation type magnetic head having an inductive head for recording and a magnetoresistive head for reproducing in combination, a means for moving the magnetic head relative to the magnetic recording medium and a read/write signal processing means for processing input signals to the magnetic head and output signals from the magnetic head wherein, in the magnetic recording medium, the seed layer comprises an amorphous alloy or a microcrystal alloy containing Ni, Ta and Zr, one or plurality of underlayers comprises an alloy containing Cr as a main ingredient and containing Ti and one or plurality of magnetic layers comprise a first magnetic layer in contact with the underlayer and a second magnetic layer formed on the first magnetic layer, and the first magnetic layer comprises a Coxe2x80x94Crxe2x80x94Pt alloy having a substantially HCP structure and the second magnetic layer comprises a Coxe2x80x94Crxe2x80x94Ptxe2x80x94B alloy or Coxe2x80x94Crxe2x80x94Ptxe2x80x94Ta alloy having a substantially HCP structure.
The underlayer of the magnetic recording medium serves to orient the C axis of the magnetic layer using the Co alloy substantially of the HCP structure within a plane of film and decrease the grain size of the magnetic layer. When a CoCrPt alloy at high Pt concentration is used for the magnetic layer, since customarily used Cr underlayer causes a problem in the lattice matching, it is effective to use a Crxe2x80x94Ti alloy substantially of an BCC structure as disclosed in Japanese Patent Laid-open No. Sho 62-257617. Since the Crxe2x80x94Ti alloy has a larger lattice spacing compared with Cr, it has favorable lattice matching with the CoCrPt alloy at high Pt concentration and the grain size can be decreased.
However, as the Ti concentration increases, this involves a problem of weakening (001) orientation which is a desired crystallographic orientation for the underlayer. The present inventors have studied on various materials as a seed layer formed between the substrate and the underlayer and have found that, when an NiTaZr alloy is used as the seed layer, (001) orientation in the CrTi alloy underlayer is increased and the grain size in the underlayer can be made smaller. Since the NiTaZr alloy shows no distinct diffraction peak in X-ray-diffraction and shows neither distinct diffraction spot nor diffraction ring also in electron diffraction, it is considered that the alloy is in an amorphous or microcrystal form. For the composition of the NiTaZr alloy, it is desirable that the Ta concentration is 30 at % or more and 60 at % or less and the Zr concentration is 5 at % or more and 20 at % or less. If the Ta concentration is out of the range described above, it is not desirable since crystallization of the NiTaZr alloy sometimes occurs depending on the film deposition condition and (001) orientation of the CrTi alloy underlayer is deteriorated. Further, if the Zr concentration decreases to 5 at % or less, although the CrTi alloy underlayer has strong (001) orientation, the grain size of the underlayer is increased. On the other hand, if it is 20 at % or more, (001) orientation is undesirably degraded.
The magnetic layer of the magnetic recording medium is constituted with a first magnetic layer in contact with the underlayer and a second magnetic layer formed on the first magnetic layer, in which a CoCrPt alloy can be used as the material for the first magnetic layer and a CoCrPtTa alloy or CoCrPtB alloy can be used as the material for the second magnetic layer. Particularly, the CoCrPtB alloy tends to give a high Hc even if the grain size is small, which is desirable in view of improving the output resolution. Further, as the second magnetic layer, a magnetic layer of a so-called granular structure comprising a Coxe2x80x94Pt alloy of high magnetic anisotropy and an oxide (SiO2, Al2O3, etc) can be used. Use of the single layer of the CoCrPt alloy for the magnetic alloy is not preferred since the overwrite characteristics tend to be deteriorated with a composition at high Pt concentration.
When an intermediate layer comprising a CrMo alloy is formed between the magnetic layer and the underlayer, a single layer of a CoCrPtTa alloy or CoCrPtB alloy can be used as the magnetic layer. Further, it is also possible to use a magnetic layer of the granular structure comprising a CoPt alloy and an oxide. Particularly, use of the single layer of the CoCrPtB alloy tends to give fine grain size and high Hc simultaneously and low medium noise and high resolution performance can be attained desirably. When a CoCrPtTa alloy or CoCrPtB alloy magnetic layer is formed directly on the CrTi alloy underlayer, epitaxial growth is difficult in a composition region in which the Cr and the Pt concentrations are high. However, the C axis of the CoCrPtTa alloy or CoCrPtB alloy magnetic layer can be oriented in-plane by using the CrMo alloy intermediate layer. Since the CrMo alloy is a complete series of solid solution, lattice matching with the CoCrPtTa alloy or CoCrPtB alloy magnetic layer can be enhanced by controlling the concentration of Mo having larger atomic radius compared with Cr. Further, since the grain size of the CrMo alloy tends to increase with the thickness, it has to be used within a range of a reduced thickness. The thickness of the intermediate CrMo alloy layer is preferably from 3 nm or more and 10 nm or less for keeping good crystallographic orientation and suppressing the growth of the crystal grain.
It is necessary to use a substrate of excellent surface smoothness and, specifically, an Alxe2x80x94Mg substrate having NiP formed on the surface, glass substrate, SiO2 substrate, SiC substrate and carbon substrate can be used. The Alxe2x80x94Mg substrate is usually applied with texturing on the surface and provided with the magnetic anisotropy in the circumferential direction of the substrate. In a substrate for which application of mechanical texturing is difficult such as a glass substrate, the magnetic anisotropy can also be provided in the circumferential direction of the substrate by applying slight texturing of about Ra=1 nm after forming a seed layer. As the protection film for the magnetic layer, a film mainly composed of carbon as a main ingredient at a thickness of. the 3 nm or more and 12 nm or less is formed, and a lubrication layer such as of perfluoroalkyl polyether or the like is formed at a thickness of 1 nm or more and 10 nm or less, to obtain highly reliable magnetic recording medium.
A magnetoresistive sensor of a magnetoresistive head for reproducing used in the magnetic storage apparatus according to this invention is formed preferably between two shield layers made of a soft magnetic material spaced apart from each other at a distance of 0.12 xcexcm or more and 0.18 xcexcm or less. If the distance between the shield layers is larger than 0.18 xcexcm, it is not preferred since the resolution is lowered. On the other hand, if it is smaller than 0.12 xcexcm, it is not preferred since the insulation property between the shield layer and the magnetoresistive sensor may possibly be degraded. Further, when the magnetoresistive head is constituted with a plurality of conductive magnetic layers of causing great resistance change by relative change of the magnetization direction to each other by external magnetic fields and a magnetoresistive sensor including a conductive non-magnetic layer disposed between the conductive magnetic layers, the reproducing signal can be increased and a magnetic storage apparatus of high reliability at a recording density of 10 Gbits or more per one square inch can be obtained.